1. Field of the Invention
The present invention relates to a flow amount measuring apparatus for measuring flow amount of fluid.
2. Description of Related Art
JP-A-2000-193505 discloses a flow amount measuring apparatus. The flow amount measuring apparatus has a temperature sensitive resistor that is disposed upstream a heating resistor. The temperature of the temperature sensitive resistor varies in accordance with flow amount of fluid and flow direction of fluid, and the temperature can be detected as resistance thereof. Therefore, the temperature sensitive resistor can detect both the flow amount of fluid and the flow direction of fluid.
According to this technique, the temperature of the temperature sensitive resistor varies within a small range. For this reason, it is required to enhance an amplitude ratio for amplifying detected voltage. On the other hand, in order to reduce difference of output characteristics between products, it is also required to adjust the amplitude ratio. However, output may be varied significantly in response to a very small amount of adjustment. Therefore, it was not easy to adjust the output.
JP-A-9-243423 discloses a flow amount measuring apparatus. The flow amount measuring apparatus has a first temperature sensitive resistor that is disposed upstream a heating resistor and a second temperature sensitive resistor that is disposed between the first temperature sensitive resistor and the heating resistor. According to this technique, it is possible to detect flow amount of fluid based on a difference between temperatures of the first temperature sensitive resistor and the second temperature sensitive resistor.
In this technique, it is also required to use great amplitude ratio. Therefore, it has the same disadvantages discussed above.
In addition, as a method for adjusting the amplitude ratio, a laser trimming method is available. In this method, resistance can be trimmed by applying laser beam directly onto a resistive layer formed on a chip. However, due to an expensive laser trimming machine, product price is also increased. Moreover, the laser trimming must be performed before attaching a cover and the like, that is, before completing a manufacturing of product. Therefore, the output characteristic may deviate during manufacturing process after the trimming process.
In addition, an electrical trimming method for electrically performing the trimming is also known. The electrical trimming method is effective if a temperature of fluid is constant. The output may be varied in response to change in the temperature of fluid. For example, in an airflow meter for detecting flow amount of intake air of a vehicular engine, the temperature of fluid varies frequently. For such purpose, the electrical trimming method has a disadvantage that it is not able to measure flow amount accurately.